The present invention relates to a nickel mixed hydroxide with Ni as the main element and with an expanded layer structure, a process for the preparation thereof by co-precipitation of the hydroxides in an alkaline medium and to the use thereof as cathode material in alkaline batteries.
β-Nickel(II) hydroxide is used in alkaline accumulators as positive electrode material. Changes in certain electrochemical properties may be obtained by incorporating foreign ions.
The incorporation of trivalent ions in the nickel hydroxide matrix in molar proportions of >20 mol % leads to a new structure. The materials thus altered have the structure of hydrotalcite and, in comparison with β-Ni(OH)2, are characterised by an expanded layer structure in the intermediate layers of which water and various anions are present. The layer expansion alone has a fundamental influence on the electrochemistry, in this case on the potential position and electrochemical usefulness of the nickel ions. The trivalent cation used in each case exerts an additional effect on the electrochemical behaviour of the materials.
Single-substituent variants containing the substituents Fe, Mn, Co and Al are known from the literature. Most have improved utilisation of the nickel ions but their stability is not very pronounced. Others, on the other hand, have good cycle stability but the nickel utilisation is lower.
Substitution with a combination of two different cations is also found in the literature. EP 0 793 285 A1 describes nickel hydroxide materials which contain, for example, the elements Co or Mn in combination with elements such as, e.g., Fe, Al, La and others. Co and Mn are used in divalent form in the preparation of materials, no oxidising agents being used in the further course of preparation. Where preparation takes place by electrochemical (cathodic) deposition, precipitation even lakes place in a reducing environment due to accompanying hydrogen evolution. Co is therefore present in the divalent form in the finished active material, together with trivalent cations such as Fe, Al, La amongst others The materials are analysed and assessed in terms of their potential position during the discharge process and charge acceptance at relatively high temperatures, but there are no details whatsoever about the cycle stability and actual electrochemical utilisation in the form of absolute values. Only relative capacities are given.
EP 0 712 174 A2 describes nickel hydroxides in which, in contrast to EP 0 793 285 A1, trivalent instead of divalent Mn ions are used as substituents in combination with other trivalent elements such as Al, Fe and Co. Mn is present in the product in the trivalent form together with Al, Fe or Co, and it is also possible for Mn to be present in several oxidation states simultaneously (“mixed valent”) in the solid. These Mn-containing materials exhibit pronounced cycle stability but the nickel utilisation is only slightly above that of the conventional nickel hydroxides. No practical indications that the use of mixed valent systems other than Mn may lead to an improvement in the capacity and nickel utilisation can be derived from this prior art.
The object of the present invention is to provide a nickel mixed hydroxide which, whilst having good cycle stability, exhibits a marked improvement in the electrochemical utilisation of the nickel ions and the mass-related capacity. Moreover, a simple process for the preparation of such a nickel mixed hydroxide should be given.
This object is achieved by a nickel mixed hydroxide according to claim 1. Advantageous embodiments of the nickel mixed hydroxide according to the invention are given in subclaims 2 to 6.